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10
SERIES
The choice of frequency is dependent on the conditions of the locate. It is generally accepted that
lower frequencies provide a more accurate locate. Therefore it is generally advised to use the lowest
frequency that effectively conducts on the target line. Lower frequencies are less likely to bleed onto
adjacent non-target lines, but low frequencies are less effective at locating higher-resistant conductors.
FREQUENCY RANGES
Frequencies are classified into three major categories, Low, Medium and High and they are classified
as follows:
• Low Frequencies
< 1 kHz (1000 Hz)
e.g. 512 Hz, 640 Hz, 815 Hz
• Medium Frequencies 1 kHz to 44 kHz
e.g. 4 kHz, 8 kHz, 33 kHz
• High Frequencies
44 kHz to 480 kHz
e.g. 65 kHz, 82 kHz, 200 kHz
FREQUENCY CHARACTERISTICS
The low, medium and high frequencies each have their advantages. It is recommended to begin by
using a low frequency, and continue as long as you are confident in the results. If the signal is very
weak try to adjust the connection or grounding. If there is no improvement in signal then try mid-
range frequencies. Again, if there is weak or absent signal, repeat adjustments of the grounding and
connection point before switching to a high frequency.
The method of signal application may dictate the frequencies used. While direct connection can be
performed on any frequencies, coupler induction will only work with medium and high frequencies
and transmitter induction will only work on the highest of the medium frequencies (33kHz) and high
frequencies.
Low Frequencies
are usually preferred to the mid-range frequencies and high frequencies because
they are less susceptible to locating errors caused by coupling of signal to adjacent conductors, cables
or pipes. The low frequency locating range is also much longer than the medium and high frequencies.
Low frequencies will not travel well on highly resistive lines or conductors that have interruptions in
their continuity (e.g. disconnected shield bonds or insulated pipe bushing). Low frequencies may only
be applied via direct connection.
Medium Frequencies
take the best of both high and low frequencies. Medium frequencies are not as
susceptible to bleed off or coupling as the high frequencies, and medium frequencies can jump minor
breaks in continuity and conduct on higher resistant lines better than low frequencies. It is still best to
use low frequencies whenever effective, but 8 kHz and 33kHz are of the most common frequencies
used in locating. Medium frequencies may be applied via direct connection and via coupler induction
[Note: 33kHz may be applied via transmitter induction].
High frequencies
are sometimes better than the low frequencies for locating highly resistant lines as
well as conductors that have breaks in their continuity (e.g. disconnected shield bonds or insulated
pipe bushing). The locating range is quite short for the high frequencies so the Transmitter must be
repositioned more often during the tracing operation. High frequencies may be applied via direct
connection, coupler induction and transmitter induction.
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